One of the main objects in nowadays's oil refining is to produce gasolines fulfilling the increasing environmental demands on product quality and having a high octane number.
This means for gasoline that the octane specification has now to be established without lead-containing additives, less aromatics, in particular benzene, less olefins and lower gasoline vapor pressure.
Components which are suitable for improving the octane quality of gasoline are certain highly branched ethers, e.g. methyl tertiary butyl ether (MTBE). It is known that such ethers can be formed by contacting branched olefins, e.g. branched butenes, with methanol in the presence of a suitable acidic catalyst, such as sulphonic resins, phosphoric acid, modified kieselguhr, silica/alumina and acidic zeolites.
A process for preparing branched butenes has for instance been described in U.S. Pat. No. 5,146,035. It discloses that in a process for the isomerization of butenes, a catalyst comprising a metal aluminophosphate as described in general terms in U.S. Pat. No. 4,567,029 can be used (MeAPO).
Surprisingly, it has now been found that an improved yield of branched olefins can be obtained if a hydrocarbonaceous feedstock comprising linear olefins having at least four carbon atoms is contacted with a catalyst comprising a medium-pore molecular sieve selected from a specific group of MeAPO and MeAPSO medium-pore molecular sieves.